Add version support to docs site

[nguyer]

This PR adds significant new functionality to the docs GitHub action to manage versions of the docs site. This will create a directory tree that ends up looking like this:

+ / (latest release)
|-- v1.0.0
|-- v1.0.1
|-- ...
|-- head (current commit in main)

It will also build a JSON file that contains the full list of all these versions to build the dropdown menu that shows up in the docs rendered pages (with the addition of a latest link that links to /).

These changes allow us to always keep the docs up to date with the code in the same commit, but users navigating to the docs site will see the latest release by default. They can also use the dropdown menu to browse older release as well.

NOTE There will be a one time manual action necessary to generate docs for older versions. I can take care of this after this PR is approved and merged.

[codecov-commenter]

Codecov Report

Merging #830 (80bcd71) into main (0e4c169) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #830   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          299       299           
  Lines        19457     19457           
=========================================
  Hits         19457     19457           

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[peterbroadhurst]

Approved - as this looks sound, and I could follow the logic.
One item to review with regards to future maintainability.